1. Field of the Invention
This invention relates to an on-load tap-changing transformer, and more particularly to that of the coarse tap selector type provided with a coarse tap coil and a fine tap coil.
2. Description of the Prior Art
An on-load tap-changing transformer having a structure as shown in FIG. 1 is known as one form of such a transformer. Referring to FIG. 1, a high-voltage winding 1 has a high-voltage line terminal 6 terminating in the middle thereof and is divided into halves in its axial direction (in the direction of its height) at the connected position of this terminal 6. The halved portions of the high-voltage winding 1 are connected in parallel with each other. A coarse tap coil 2 and a fine tap coil 3 having approximately equal inductance values are connected to each of the upper and lower ends of the high-voltage winding 1, and a low-voltage winding 4 is disposed inside the high-voltage winding 1. The reference numeral 5 designates the iron core of the transformer.
In the present application, the terms "minimum-voltage tap selection mode", "rated voltage tap selection mode" and "maximum voltage tap selection mode" are used hereinafter to designate the case in which the tap coils 2 and 3 are disconnected from the circuit, and primary current flows through the high-voltage winding 1 only; the case in which the fine tap coils 3 are disconnected from the circuit, and primary current flows through the coarse tap coils 2 and high-voltage winding 1; and the case in which primary current flows through all of the coarse tap coils 2, fine tap coils 3 and high-voltage winding 1, respectively.
In the transformer structure shown in FIG. 1, primary current flows through the high-voltage winding 1 only in the minimum voltage tap selection mode in which the coarse and fine tap coils 2 and 3 are disconnected from the circuit to render the turns of the primary winding to the minimum. In such a mode, no magnetomotive force is produced in the tap coils 2 and 3 in the axial direction of the high-voltage winding 1, and there occurs an increase in the magnetic flux leaking from the upper and lower end portions of the high-voltage winding 1 in the radial direction of the high-voltage winding 1. That is, the quantity of leakage flux increases, resulting in an increased stray loss. Especially, when the transformer has a large capacity, the electromagnetic action between the radially leaking magnetic flux and the short-circuit current appearing in a shorted condition generates a large mechanical force in the axial direction of the primary winding, tending to impair the primary winding. For the above reason, the transformer structure shown in FIG. 1 is not applicable to a transformer of large capacity.
An on-load tap-changing transformer having a structure as shown in FIG. 2 is proposed to obviate the defect pointed out above. In the transformer structure shown in FIG. 2, the coarse tap coils 2 and fine tap coils 3 are disposed at a position radially spaced apart from the high-voltage winding 1, and the high-voltage winding 1 has a height (the axial level) equal to that of the low-voltage winding 4. Therefore, even in the minimum voltage tap selection mode in which all of the coarse tap coils 2 and fine tap coils 3 are disconnected from the circuit, no unbalance occurs between the axial magnetomotive forces of the high-voltage and low-voltage windings 1 and 4. The defect described with reference to FIG. 1 can thus be obviated, and the transformer structure shown in FIG. 2 is applicable to a transformer of large capacity. However, due to the fact that the coarse tap coils 2 and fine tap coils 3 are spaced apart from the high-voltage winding 1 in the radial direction, the transformer structure shown in FIG. 2 is defective in that the increase in the radial dimensions of the primary winding lowers the space factor of the primary winding.
An improved on-load tap-changing transformer as shown in FIG. 3 is proposed to obviate the defect of the transformer shown in FIG. 2. In FIG. 3, the coarse tap coils 2 are disposed at the upper and lower ends respectively of the high-voltage winding 1, and the fine tap coils 3 only are disposed at a radial position spaced apart from the high-voltage winding 1. According to the transformer structure shown in FIG. 3, the unbalance between the axial magnetomotive forces of the high-voltage and low-voltage windings 1 and 4 in the minimum voltage tap selection mode is relatively small. Therefore, the illustrated transformer structure is satisfactorily applicable to a transformer of large capacity. Also, because of the fact that the fine tap coils 3 only are disposed at the radial position spaced apart from the high-voltage winding 1, the radial dimensions of the primary winding do not appreciably increase, and the space factor of the primary winding is not also appreciably lowered.
However, in the transformer structure shown in FIG. 3, the electrostatic coupling between the end of the coarse tap coils 2 and that of the fine tap coils 3 is not strong. Therefore, in the event of intrusion of a lightning impulse voltage from the high-voltage line terminal 6, a high voltage appears across the tap coils 2 and 3, and the withstand voltage characteristic of the electrodes in the diverter switch of the on-load tap changer becomes especially a serious problem.
This problem will be described in further detail with reference to FIG. 4 which is a tap connection diagram of the transformer shown in FIG. 3. Referring to FIG. 4, the on-load tap changer generally designated by the reference numeral 7 includes a coarse tap selector 8, tap selectors 9A, 9B and a diverter switch 10. The diverter switch 10 includes a pair of electrodes 11A and 11B. In FIG. 4, the coarse tap coil 2 is connected to the other terminal or the neutral point of the high voltage winding 1, and the fine tap coil 3 is connected through the coarse tap selector 8 to the coarse tap coil 2. Eight taps T.sub.1, T.sub.2, . . . , T.sub.8 of these tap coils 2 and 3 are alternately changed over by the tap selectors 9A and 9B. More precisely, the odd-numbered taps T.sub.1, T.sub.3, T.sub.5 and T.sub.7 are sequentially selected by the tap selector 9A, and the even-numbered taps T.sub.2, T.sub.4, T.sub.6 and T.sub.8 are sequentially selected by the tap selector 9B. Thus, when one of the tap selectors selecting one of the associated taps comes to the position of the last tap in the array, the other tap selector returns to the position of the first tap in the array as shown in FIG. 4. Such a selection sequence is repeated thereafter.
In the on-load tap-changing transformer having such a structure, the voltage appearing across the electrodes 11A and 11B of the diverter switch 10 is normally equal to the voltage across the adjacent taps. Since the fine tap coil 3 is disposed at the radial position spaced apart from the coarse tap coil 2, the electrostatic coupling therebetween is not strong. Therefore, in the event of application of a lightning impulse voltage including high-frequency components to the high-voltage line terminal 6, high-frequency voltages whose absolute values are approximately equal to each other are induced in the coarse tap coil 2 and fine tap coil 3 respectively. In this case, the phase of the voltage induced in the fine tap coil 3 is delayed relative to that induced in the coarse tap coil 2, since the fine tap coil 3 is remote from the high-voltage winding 1 relative to, the coarse tap coil 2. Especially, when the tap selectors 9A and 9B are connected to the taps T.sub.1 and T.sub.8 respectively as shown in FIG. 4, a large voltage differential attributable to the phase difference is applied across the electrodes 11A and 11B of the diverter switch 10 although the absolute values of the voltages induced in the tap coils 2 and 3 may be the same. This voltage differential is so large that it is substantially equal to the voltage induced across the most spaced taps. Therefore, the tap arrangement in such a transformer is limited by the withstand voltage characteristic of the electrodes 11A and 11B of the diverter switch 10 employed in the transformer, and, because of such a limitation, the transformer structure shown in FIG. 3 is not applicable to a transformer of, for example, insulation grade No. 170 or higher in which the insulation grade of the high-voltage line terminal is very high.
FIG. 5 shows schematically in section the arrangement of the conductors in the fine tap coil 3. Generally, the number of required conductors constituting the fine tap coil 3 is odd when the fine tap, coil 3 is combined with the on-load tap changer 7 including the coarse tap selector 8 therein. Thus, when, for example, seven conductors a, b, c, d, e, f and g are wound in a relation superposed in double layer form in the axial direction of the high-voltage winding 1 as shown in FIG. 5, an electrical insulator 12 is used to shape up the external configuration of the fine tap coil 3. In this manner, the seven conductors a, b, c, d, e, f, g and the insulator 12 are superposed in double layer form in the direction of height to be wound together into a cylindrical-helical form as shown by a block 13 in FIG. 5. Another block 14 indicates a repetition of the block 13, and, therefore, it is not shown in detail. Such a prior art conductor arrangement is defective among others in that the conductor winding operation is troublesome since an especially prepared insulator 12 must be used for the production of the fine tap coil 3.
FIG. 6 shows the distribution of the magnetomotive force generated at various principal tap positions in the on-load tap-changing transformer having the structure shown in FIGS. 3 and 4. The thick solid curve LV in FIG. 6 represents the distribution of the magnetomotive force induced in the low-voltage winding 4. The magnetomotive force in the middle portion of the distribution curve LV is smaller than that in the remaining portions because the middle portion of the low-voltage winding 4 is coarsely wound. In the minimum voltage tap selection mode, the distribution of the magnetomotive force of the primary winding is represented by the one-dot chain curve HV.sub.L. Since, in this mode, primary current does not flow through the tap coils 2 and 3 and flows only through the high-voltage winding 1, the magnetomotive force is zero in the upper and lower end portions of the primary winding as seen in FIG. 6. In the case of the curve HV.sub.L too, the magnetomotive force is similarly small in the middle portion of the distribution curve HV.sub.L because the middle portion of the high-voltage winding 1 is also coarsely wound. The dotted curve HV.sub.R represents the distribution of the magnetomotive force in the rated voltage tap selection mode in which primary current flows through the coarse tap coils 2 too but not through the fine tap coils 3. Since, in this mode, the total primary current decreases due to the insertion of the coarse tap coils 2 in the circuit, the magnetomotive force of the high-voltage winding 1 is smaller than that in the minimum-voltage tap selection mode, and the magnetomotive force generated by the coarse tap coils 2 appears in the upper and lower end portions of the primary winding as seen in FIG. 6. The thin solid curve HV.sub.H represents the distribution of the magnetomotive force in the maximum-voltage tap selection mode in which primary current flows also through the fine tap coils 3. Although, in this mode, the magnetomotive force of the high-voltage winding 1 is smaller than that in the rated voltage tap selection mode, the magnetomotive force appearing in the upper and lower end portions of the primary winding is larger than that in the rated voltage tap selection mode because the magnetomotive force of the fine tap coils 3 is added to that of the coarse tap coils 2 in such end portions. It can be understood from FIG. 6 that the prior art on-load tap-changing transformer having the structure shown in FIGS. 3 and 4 is defective among others in that the magnetomotive force is zero in the end portions of the primary winding in the minimum voltage tap selection mode.